This invention relates to an interferometer for measuring deformation or geometry of an object with precision, utilizing the wavelength of light, and more particularly to such an interferometer that is configured to facilitate detection of alignment with a moire pattern displayed on a monitor screen.
An interferometer, as illustrated in FIG. 1, for instance, is used to measure a degree of deformation of an object with precision, utilizing the wavelength of light. In the drawing, numeral 1 indicates a source of laser beam, 2 a relay lens, 3 an imaging lens, 4 a half mirror, 5 a surface to be measured, 6 a reference surface with an adjustable angle, 7 an image sensor receiving interference fringes, and 8 a monitor for displaying interference fringes.
When a laser beam is emitted from the laser beam source 1, it is passed through the relay lens 2 and reaches the half mirror 4 where it is separated into two beam rays. One of the beam rays then reaches the surface 5 to be measured, on which it is reflected and reaches the half mirror 4 again. The other beam ray, separated by the half mirror 4, reaches the reference surface 6, on which it is reflected to reach the half mirror 4 again. The beam ray reflected on the reference surface 6, and that reflected on the surface 5, interfere with each other so that interference fringes are formed on the image sensor 7 through the imaging lens 3. Interference fringes thus appear on the screen of a monitor 8.
When the reference surface 6 is tilted, a number of interference fringes L, as illustrated in FIG. 2, are seen on the monitor 8. The spatial fringe scanning method is applicable to such a case as one of high-precision fringe analyzing algorithms to enable a degree of deformation of an object to be calculated, from such interference fringes L with, utilizing the precision under wavelength of light.
The number of interference fringes L is set at 64 in view of the fringe analyzing algorithm and the number of picture elements on the screen of the monitor 8. Consequently, the tilt angle .theta. of the reference surface 6 is adjusted to provide 64 interference fringes for alignment of the interferometer.
In the device as mentioned above, however, alignment of the interferometer can be detected by checking whether the number of interference fringes seen on the monitor 8 is 64 or not, that is, by counting the number of interference fringes on the monitor 8 one by one. The detecting operation has therefore suffered very bothersome and inefficient procedures.
An alternative arrangement has therefore been proposed, wherein an optical grid image is formed on the surface to be measured, which is received by a moire pattern generator to generate a moire pattern that is used to measure deformation of an object. The actual constitution is, for instance, as illustrated in FIG. 3.
The arrangement shown consists of a projection optical system 31 for projecting a grid image onto an object H to be measured and a moire pattern generator 32 which receives the grid image reflected from the object H to generate a moire pattern. The projection optical system 31 consists of a luminous source R, a relay lens 33, a first reference grid 34, and a relay lens 35. The moire pattern generator 32 consists of an objective lens 36, a second reference grid 37, a relay lens 38, an imaging lens 39 and a film F.
When a light is emitted from the light source R, the light passes through the relay lens 33, the first reference grid 34 and the relay lens 35 to reach the object H, where a grid image is formed by means of the first reference grid 34. The reflected grid image then reaches the second reference grid 37 through the objective lens 36 of the moire pattern generator 32. The image of the first reference grid 34, which has been deformed in dependence on the geometry of the object H, is then laid on the second reference grid 37, producing a moire pattern. The moire pattern is developed on the film F by means of the relay lens 38 and the imaging lens 39. The geometry of the object H is thus measured in accordance with the moire pattern formed on the film F.
The moire pattern generator 32 as described above, however, has drawbacks in that, since the second reference grid 37 consists of a substrate having numerous slits, it is difficult to change the number of the slits and their pitch as desired, and that contrast of the moire pattern is undesirably low.
One example proposed to overcome these problems is given by an aspherical surface inspection device such as illustrated in FIG. 4, which has been described in "KOGAKU" Vol. 16, No. 4 (Apr. 1987) published by the Optics Branch of the Society of Applied Physics, in Japan.
A moire fringe image signal generator 49 (FIG. 4) comprises a CCD (Change Coupled Device) 54, a piezo element 55, a microprocessor 56, an image memory 58, a processor 59, and a monitor 60. An interference optical system 50 comprises a laser light source 51, a half mirror M, a reference mirror 53, and a set of relay lenses 64a to 64c. The microprocessor 56 calculates the intensity distributions of interference fringes per phase variation of the reference light by 2/N in accordance with the assumption that a surface of the object 52 to be measured has zero geometrical error. The N distributions of interference fringe intensity are stored, in advance, in N (four in FIG. 4) image memories 58.
When a laser beam is now emitted from the laser light source 51, it is passed through the relay lens 64a to reach the half mirror M where it is separated into two beam rays. One of the beam rays is passed through the relay lens 64b to the object 52, on which it is reflected and reaches the half mirror M via the relay lens 64b again. The other beam ray separated by the half mirror M reaches the reference mirror 53, on which it is reflected to reach the half mirror M again. The laser beam reflected on the reference mirror 53 and that reflected on the object 52 interfere with each other so that interference fringes are formed on the CCD 54 through the relay lens 64c.
The reference mirror 53 is attached to the piezo element 55 so that the phase of the reference light is varied by 2/N (N is an integer) per 1/30 second in synchronism with the timing of image receipt timing at the CCD 54.
The processor 59 serves to operate, for each phase variation of the reference mirror 53, the moire fringe image signals corresponding to the moire pattern generated by the interference between the actual interference fringes formed on the CCD 54 and the virtual computer hologram (interference fringe intensity distribution) which consists of the reference grid stored in the image memory 58. It then calculates the sum of these moire pattern images to display it on the monitor 8. It is intended to eliminate flickering of the moire pattern or to improve the contrast through such calculation.
Driver 61 drives the piezo element 55, and controller 62 controls the driver 61 and image memory 58.
However, in the device described above, there is a problem in that the piezo element 55 must be used to move the reference mirror 53 in accordance with the wavelength of light, which makes the device complicated in structure and extremely expensive.